Project Summary/Abstract Positive social support is associated with health behavior changes, particularly in weight loss. A lack of information as to what specifically influences positive social support presents opportunities to examine social support factors to understand how and why social support is effective in health behavior changes. In the proposed project, the applicant will focus on homophily, the degree to which individuals associate with similar others that is greater than chance, and its association with both support and weight loss within already- completed behavioral weight loss interventions. One weight loss intervention included a social support component, whereby participants selected a weight loss supporter (?Buddy?) who was potentially randomized to receive formal training (?Buddy training?) on how to provide support. In this study, a moderated regression analysis will examine the relationship between Buddy training and participant weight loss, moderated by the degree of homophily among weight loss participants and their Buddies. To supplement these analyses, semi- structured qualitative interviews with participants who completed the weight loss intervention and whose Buddies underwent Buddy training will provide additional information about how social support was perceived, and whether homophily was recognized by participants as an intentional aspect of their decision-making process when selecting their Buddy. Then, within a group-based weight loss intervention, where group members did not know each other prior to study start, regression analyses will examine the effect of overall group homophily on individual weight loss at follow-up assessments. Finally, the potential mediating role of group cohesion and autonomy support will be examined. If the proposed project is awarded, the applicant's research will be supplemented by elective coursework in qualitative methods and advanced statistics, clinical work in populations affected by obesity, and professional development opportunities in behavioral medicine to promote growth toward becoming an independent researcher.